In waste-burning plants, as we know, combustion residues occur in the form of slag, flyash and filter dust, which are relatively strongly loaded with substances harmful to the environment. The slag occurring in rubbish and waste combustion ovens constitutes, by amount, the greatest part of the combustion residues. The slag has, on leaving the combustion plant, contents in harmful substances which prevent its further use, for example, as building material, and even the storage of the slag in dumps can be an environmental hazard. The content in harmful substances depends on the composition of the rubbish or waste delivered for burning and also on the quality of the combustion process On an average, the content in harmful substances of slags from household waste burning plants should be in the range from 50 to 100 grams per kilogram slag. The harmful substances contained in the slag are, in particular, chlorides, lead, cadmium, zinc, iron, copper, organic substances and other impurities. Corresponding analyses have been published, for example, in the publication Swiss Engineer and Architect, No. 19, 1983. In the storage of slags from waste-burning plants in dumps, the leaching water must be collected and decontaminated, since this is otherwise a considerable danger to the environment. Corresponding problems occur also with the use of slag in road building. Therefore, in these cases, the slag is very often mixed with cement and made impermeable by water, or stabilized. However, the organic substances, especially carbon compounds, decompose the slag in a long-term process, so that harmful substances can emerge even with stabilized slags. Since dumps for slag are becoming less available all the time, the storage of slag presents a great problem. The use in road building is also greatly limited by the high content in harmful substances, and the danger of decomposition in the isolated fields of use.
German Publication No. 3,700,608 describes a process and an apparatus for treatment of slag in which, after chilling the slag in the known wet slagger or water bath, fines with a grain size up to 1 mm are removed from the slag. The remaining slag has a reduced content in harmful substances and can be used, under certain preconditions, for street and road building. This known process for cleaning the slag still cannot satisfy the increasing claims to the minimum content of harmful substances in the slag. The cleaned slag still contains residues of water-soluble harmful substances, and also of organic substances which decompose the slag. The chloride content, for example, may be up to 50% of the original amount. Another disadvantage is that the slag must be temporarily stored after chilling in the water bath, and thus considerable storage volumes are necessary. The dripping water occurring must also be contaminated since it is fouled. The harmful substances of the dust part are floated by the chilling bath into the slag, and adhere in the pores after drying. The proposed drying to a residual moisture of 1% requires, with great amounts of slag, a very high consumption of energy. The cleaned slag with this high degree of drying is not suitable as a building material and must be moistened again, for example, for road and street building.